In lithographic printing, an inked printing member, e.g., an inked printing plate, contacts and transfers an inked image to a rubber blanket, and then the blanket contacts and transfers the image to the substrate being printed. Lithographic plates have image areas with a relatively oleophilic surface and non-image areas with a relatively hydrophilic surface. In a typical two-fluid lithographic printing process, the plate cylinder first comes in contact with dampening rollers that transfer an aqueous fountain solution that wets out the hydrophilic non-image areas of the plate. The dampened plate then contacts an inking roller, accepting the ink only in the oleophilic image areas. The fountain solution helps to keep the ink out of the non-image areas. The press operator must continually monitor the printing process to insure that the correct balance of the fountain solution and the ink is maintained so that the ink adheres to the printing areas, but only the printing areas, of the plate in order to produce a sharp, well-defined print.
Printing plates can be made by altering the ink-receptiveness of a blank plate with digitally controlled equipment to produce the desired image on the plate. Typical two-fluid lithographic printing plates typically have non-image areas that are metallic with an oxidized, hydrophilic surface. A finisher is applied over the surface of the plate to provide temporary protection. The imaged plate is then attached to the plate cylinder of the printing press. The finisher is removed during the make-ready process to re-expose the oleophilic image and hydrophilic non-image regions of the plate. Sometimes a print run is interrupted or a plate will be re-used at a later time. In such cases, a protective material, or gum, is applied to the plate surface to avoid damage to the plate and contamination with substances (dirt, oil, fingerprints, etc.) that could affect the characteristics of the hydrophilic and oleophilic areas. Like the finisher, the gum is applied in a layer over the entire surface of the plate. Typically, a gum such as gum arabic is used with two-fluid lithographic inks, which washes away during the press run. Sometimes a gum is included in the fountain solution to replace the gum that is worn away by the operation of the press. If the press is stopped during the run, the gum may provide a minor amount of protection to the plate.
The industry has long sought an offset printing process and associated materials that would not require a separate fountain solution. Waterless plates have been made by applying to the non-image area a silicone rubber, which has a very low surface energy and is not wetted by the ink. The silicone-modified plates are expensive, however, and require expensive, specially-cooled press equipment because the fountain solution of the traditional two-fluid method also serves as a coolant. Other efforts have been directed to producing a single-fluid lithographic ink, i.e., an ink that does not require a separate fountain solution, that can be used with the industry-standard printing plates that have metallic non-image surfaces.
Kingman et al., U.S. Pat. No. 6,140,392, describe single fluid lithographic printing inks having a continuous phase that includes an acid-functional vinyl resin and a discontinuous polyol phase that includes a liquid polyol. Latunski et al., co-pending U.S. application Ser. No. 10/054,261, filed Jan. 21, 2002 describes single fluid lithographic printing inks having a continuous phase including a hydrogen-bonding vinyl polymer and an emulsified phase including water and/or liquid polyol. These compositions are stable in the fountain and print lithographically without toning and without use of a separate fountain solution. It has been found, however, that for conventional plates that have metallic non-image areas, a single fluid in will, over time, remove any gum or other material used to treat the non-image areas of such plates. After the gum is removed, the ink may begin to tone. The toning is believed to be caused by the non-image areas no longer being hydrophilic enough to create sufficient separation of the continuous phase and the emulsified fluid phase of the single fluid ink. While conventional dual-fluid lithographic processes may replenish the gum on the plate with a fountain solution containing a gum, this approach has been ineffective for printing single fluid lithographic inks.